Methods

• 4 Nursery grounds were surveyed from June-September in

2008 and 2009 (Fig.1).

• Juvenile plaice and dab were collected using a beam trawl.

The diet of 651 fish was analysed and prey were counted

and classified into major taxonomic groups (Fig. 2 & 4).

• Macrobenthic prey samples were collected with a Van Veen

grab, washed in a 1 mm sieve and the density of major

benthic groups was calculated (Fig. 3 & 5).

• Sediment particle size analysis was carried out (Fig. 6) and

the mean sediment size (phi) was calculated.

• Organic matter was measured using the percentage loss on

ignition (LOI).

Macrobenthic prey availability in flatfish nursery grounds

and the potential for food competition between 0-group plaice and dab during ontogenetic development

F. De Raedemaecker, I. O’Connor and D. Brophy Marine and Freshwater Research Center, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland

Introduction

The quality of nursery grounds is an important factor that

determines the recruitment of juvenile flatfish to the adult

stocks (Gibson, 1994). Over the last decades, a strong decline

in commercial fisheries of plaice (Pleuronectes platessa L.) and

dab (Limanda limanda L.) in the north-eastern Atlantic region

has occurred (Millner et al., 2005). In some areas this trend

was preceded by a decline in juvenile abundances (Désaunay

et al., 2006). To evaluate anthropogenic and climate impacts on

flatfish nurseries we need a good understanding of the biotic

and abiotic factors that influence condition, growth and survival

of juvenile fish.

This study is a first step towards assessing the quality of

flatfish nursery grounds in Galway Bay. Our aim was to

describe spatial and temporal trends in the diet of flatfish

species that are sharing the same nurseries. Responses to

food availability were measured to assess the potential for

interspecific competition in relation to size-related growth of

juvenile flatfish.

Results

Less than 1 % of all fish analysed had empty stomachs and

were excluded from further analyses.

The diet of plaice and dab consisted of a wide variety of

benthic invertebrates (Table 1).

The overall dietary composition differed significantly between

species, years and nursery grounds (two-way ANOSIM).

Stomach samples from 2008 were characterized by

a lower stomach fullness,

a higher number of prey taxa,

a lower prey abundance and

a higher Shannon Wiener prey diversity

compared to samples from 2009 (two-way ANOVA).

Dab had a higher stomach fullness and a lower number of

prey taxa compared to plaice (two-way ANOVA).

0-group plaice and dab fed on similar

prey items but they showed a different

composition of dominant prey classes

with their ontogenetic development

(Fig.7).

With increasing fish size, we found a

decrease in stomach fullness

decrease in prey abundance

increase in prey diversity

(two-way ANOVA).

The dietary overlap between dab and plaice (Fig. 8)

decreased with julian date,

was higher in 2008 compared to 2009

and differed between nurseries

(Generalized Linear Model, GLM).

Nursery grounds revealed distinct macrofaunal communities

(Fig. 9) which were highly associated with the mean sediment

particle size and the depth (Correspondence Analysis, CCA).

A correlation was found between the spatial differences in

the macrobenthic communities with those in plaice and dab

diets (RELATE).

Table 1. Relative importance of major

prey taxa to the overall diet of plaice (n

= 476) and dab (n = 170), expressed as

frequency of occurrence (% F) and

numerical contribution (% N).

Fig.1. Map of Galway Bay and position of nursery

grounds

Fig.3. Collection of

macrobenthic samples

Fig.2. Collection of flatfish

Plaice

0

10

20

30

40

50

60

70

80

20-29 30-39 40-49 50-59 60-69 70-92

standard length class (mm)

nu

meri

cal

perc

en

tag

e p

rey (

%)

Dab

0

10

20

30

40

50

60

70

80

20-29 30-39 40-49 50-59

standard length class (mm)

nu

meri

cal

perc

en

tag

e p

rey (

%)

0

1 00

2 0-2 9 3 0-3 9 4 0-4 9 5 0-5 9

Bivalvia Polychaeta Malacostraca Copepoda Ostracoda Oligochaeta Others

Fig.7. Diet composition in different size classes of plaice and dab

Literature cited: Desaunay Y., Guerault D., Le Pape O. & Poulard J.C. (2006) Changes in occurrence

and abundance of northern/southern flatfishes over a 20-year period in a coastal

nursery area (Bay of Vilaine) and on the eastern continental shelf of the Bay of

Biscay. Scientia Marina: 70, 193-200.

Gibson R.N. (1994). Impact of habitat quality and quantity on the recruitment of

juvenile flatfishes. Netherlands Journal of Sea Research: 32(2), 191-206.

Millner, R., Walsh, S. J. & Díaz de Astarloa, J. M. (2005). Atlantic flatfish fisheries. In

Flatfishes: Biology and Exploitation (Gibson, R. N., ed.). Oxford: Blackwell

Science, 240-271.

Acknowledgements: This project was funded through the STRIVE Programme by the Environmental Protection Agency.

Many thanks to Stephen Comerford and Antonio Agüera García for the assistence during field sampling.

Conclusions

• Juvenile plaice and dab are opportunistic feeders but a clear distinction in diet composition

suggests a partitioning of food resources between both species.

• Interspecific competition is more likely to occur just after settling when the dietary overlap is

high, however, the density of flatfish in nursery areas did not have an effect on the dietary

overlap suggesting that there was an abundant food supply in all nursery grounds.

• The feeding efficiency, estimated by stomach fullness, was variable between years and

nurseries and this reflected differences in the productivity of the macrobenthos in the

sediment.

• Further research will be conducted to assess if the temporal and spatial differences in diet

composition and food availability affect the growth and condition of juvenile flatfish.

Plaice Dab

% N % F % N % F

Oligochaeta 0.82 4.20 13.19 37.65

polychaeta 17.41 65.97 2.00 17.06

Siphons Bivalvia 3.53 17.02 0.08 0.59

Bivalvia 12.04 53.78 9.27 47.65

Gastropoda 0.53 8.61 0.19 8.82

Amphipoda 9.11 60.08 16.44 70.00

Mysidacea 0.18 3.15 0.00 0.00

Cumacea 9.16 50.00 1.71 30.59

Copepoda 39.93 73.53 52.25 92.94

Ostracoda 5.81 40.34 3.39 38.24

Decapoda 1.26 11.97 1.40 14.71

Isopoda 0.14 1.26 0.00 0.00

Echinoidea 0.04 0.21 0.02 0.59

Arachnida 0.05 1.68 0.07 3.53

Chironomidae 0.00 0.21 0.00 0.00

Fig.4. Gut content analysis Fig.5. Sorting of macrobenthos Fig.6. Sediment particle

size analysis

Fig.8. Result of a GLM predicting the dietary

overlap between plaice and dab for different

nursery grounds in 2008 and 2009.

Fig.1. Map of Galway Bay and position of nursery

grounds

Fig.3. Collection of

macrobenthic samples

Fig.2. Collection of flatfish

Fig.9. A triplot showing the species, samples

and environmental variables in the

Correspondence Analysis